liblog: use default tag for loggability checks if no tag is provided

Bug: 116329414
Bug: 119867234
Test: new unit tests
Change-Id: I92a3f4f95e5f482f6fe20f17ed83c8ed367b06dc
diff --git a/liblog/logger_write.h b/liblog/logger_write.h
new file mode 100644
index 0000000..065fd55
--- /dev/null
+++ b/liblog/logger_write.h
@@ -0,0 +1,24 @@
+/*
+ * Copyright (C) 2020 The Android Open Source Project
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+#pragma once
+
+#include <string>
+
+#include "rwlock.h"
+
+std::string& GetDefaultTag();  // Must read lock default_tag_lock
+extern RwLock default_tag_lock;
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